The present disclosure relates generally to integrated circuits and, more specifically, to improved insulated gate field effect transistors (IGFET) in the integrated circuit.
Many of IGFET structures use a body diffusion in which the source but not the drain contact is located. The body's surface doping is one of the structure parameters that sets the threshold voltage of the device. A junction between the relatively high doped body and a lighter doped adjoining region allows a depletion layer width sufficient to support the desired voltage to expand in the lightly doped region while the high doped body prevents punch through across the body surface to a source located near to the surface intersection of the junction. This allows a short channel length.
The body layer of these devices often requires a graded profile that defines the junction with a depth on the order of or greater than 1 micron. This layer together with the region in which it is formed has a large impact on the breakdown voltage. It also controls the channel length in self aligned body structures. The surface of the body layer often requires a region of higher doping to set the threshold voltage.
In the prior art, the body has been formed by a masked implantation and diffusion. The threshold voltage setting layer has been formed by a second masked implant formed after the body diffusion. Boron has been used to form both layers in these devices. This method is illustrated in FIGS. 1A-1D. Also, indium has been used as a threshold adjustment in ≦0.18 μm CMOS processes. Indium has not been used in a lateral NDMOS application where the P type body sees significant diffusion in order to achieve the higher voltage requirement.
This disclosure describes an improved process and resulting structure that allows a single masking step to be used to define both the body and the threshold adjustment layer of the body. The method consists of forming a first mask on a surface of a substrate with an opening exposing a first region of the substrate; implanting through the opening a first impurity of a first conductivity type and having a first diffusion coefficient; and implanting through the same opening a second impurity of the first conductivity type and having a second diffusion coefficient lower than the first diffusion coefficient. The first and second impurities are then co-diffused to form a body region of a field effect transistor. The remainder of the device is formed.
The second diffusion coefficient is less than half of the first diffusion coefficient. The first impurity may be boron and the second impurity may be indium. The first impurity may include implanting the first impurity twice at more than one or at least two different energy levels. The first region may be of the second conductivity type and is the drain region. Alternatively, the first region may be of the first conductivity type and the drain region of the second conductivity type is formed in the surface of the substrate adjacent the body region.
These and other aspects of the present disclosure will become apparent from the following detailed description of the disclosure, when considered in conjunction with accompanying drawings.